1. Field of the Invention
The present invention relates to an input circuit of a television tuner.
2. Description of the Related Art
An input circuit of a conventional television tuner will be described with reference to FIG. 2. A first terminal 51 to which a television signal is input is shunted by a CB trap 52 for attenuating a CB band and is connected to an input tuning circuit 57 sequentially through an intermediate frequency (IF) trap circuit 53 for attenuating an intermediate frequency band, a frequency modulation (FM) trap circuit 54 for attenuating an FM broadcasting band, and first and second diodes 55 and 56 which are connected in series each other. The cathodes of the first and second diodes 55 and 56 are connected to each other. The first diode 55 is provided at the side of the first input terminal 51. The second diode is provided at the side of the input tuning circuit 57. The input tuning circuit 57 is switched by a diode 57a so as to be tuned to a low band and a high band of VHF bands.
On the other hand, a second terminal 61 to which an FM broadcasting signal is input is connected to the input tuning circuit 57 through an inductance element 62 and third and fourth diodes 63 and 64 serially connected to each other. The cathodes of the third and fourth diodes 63 and 64 are connected to each other. The third diode 63 is provided on the second input terminal 61. The fourth diode 64 is provided at the side of the input tuning circuit 57. The anode of the fourth diode 64 is connected to the anode of the second diode 56.
A bias voltage from a voltage dividing circuit 60 for dividing a power supply voltage B is independently applied to each of the anodes and cathodes of the first through fourth diodes 55, 56, 63, and 64.
The anode of a fifth diode 58 is connected to the anode of the first diode 55. The cathode of the fifth diode is connected to the collector of a first transistor 70. The emitter of the first transistor 70 is grounded. The bias voltage from the voltage dividing circuit 60 is applied to the collector of the first transistor. The cathodes of the third and fourth diodes are also connected to the collector of the first transistor 70. Furthermore, the anode of a varactor diode 65 is connected to the anode of the third diode 63. The cathode of the varactor diode is connected to the collector of a second transistor 71. The emitter of the second transistor 71 is grounded. The bias voltage from the voltage dividing circuit 60 is applied to the collector of the second transistor. A base is connected to the collector of the first transistor 70. The collectors of the first and second transistors 70 and 71 are high-frequency grounded by direct current cut capacitors 72 and 73, respectively.
In the above-mentioned structure, when the first transistor 70 is switched on, the second transistor 71 is switched off. Therefore, the first and second diodes 55 and 56 and the varactor diode 65 are switched off and the third and fourth diodes 63 and 64 and the fifth diode 58 are switched on. Therefore, only the FM broadcasting signal input to the second input terminal 61 is input to the input tuning circuit 57. In this case, the inductance element 62 and the varactor diode 65 may form a low pass filter.
On the other hand, when the first transistor is switched off, the second transistor 71 is switched on. Thereby, the first and second diodes 55 and 56 and the varactor diode 65 are switched on and the third and fourth diodes 63 and 64 and the fifth diode 58 are switched off. Therefore, only the television signal input to the first input terminal 51 is input to the input tuning circuit 57.
In the above-mentioned conventional structure, in order to switch the television signal or the FM broadcasting signal and to input the switched television signal or FM broadcasting signal to the input tuning circuit, a plurality of active elements (five diodes, one varactor diode, and two transistors) is used, thus increasing the cost. Also, in the case of receiving a television broadcasting which is accompanied by a video signal rather than the case of receiving FM broadcasting which is accompanied by only voice, NF (noise factor) is electrically required. Nevertheless, since the television signal is input to the input tuning circuit through the two diodes, the NF deteriorates due to loss in the television signal caused by the diodes.
Also, when receiving the television broadcasting, since it is not possible to completely block the FM broadcasting signal between the first input terminal and the input tuning circuit by the inter-terminal capacitance of the diode in which it is necessary to block the FM broadcasting signal, the FM broadcasting signal overlaps and interferes with the television signal.